1. Field of the Invention
This invention relates generally to the production of a fire-resistant cellulose insulation product, and more particularly to fire-resistant cellulose insulation materials using alkali borates as liquid flame retardants.
2. Description of Related Art
Previously, the cellulose industry used powdered boric acid (H3BO3) and powdered borax (Na2B4O7.5H2O) sodium tetraborate pentahydrate, almost exclusively as flame retardants in the manufacture of cellulose insulation. These two chemicals, mixed in the ratio range of from 1:2 to 1:4 (boric acid:borax) is still generally accepted as the best flame retardant formula although today, because of its expense, it is not commercially used. Instead, ammonium sulfate is used as a substitute for borax in a ratio range of from 1:2 to 1:6 (boric acid:ammonium sulfate). The use of ammonium sulfate has introduced problems however, such as corrosion and unacceptable odor and therefore, research is ongoing in the industry to find an inexpensive substitute for the sulfate.
All cellulose insulation must pass a series of tests described in ASTM C-739, before the U.S. Government will permit its sale. The most important of these tests are: a corrosion test; a critical radiant flux test which measures the ability of the product to prevent the spread of fire by surface burning; and a smolder test which measures the ability of the product to extinguish burning beneath the surface. The boric acid and borax mixture described above, is able to easily pass all three of these tests. Boric acid alone is able to pass the critical radiant flux, and smolder tests, but being acidic it fails the corrosion test and, when used alone it is an expensive approach. Borax alone is able to pass the critical radiant flux and corrosion tests and is cheaper than boric acid, but it does not pass the smolder test because its alkalinity, due to the sodium, actually enhances smoldering.
When combined in the preferred ratio range as defined above, the two chemicals neutralize each other so as to easily pass the corrosion test. The boric acid overwhelms the effect of the sodium ensuring that the combination passes the smolder test as well.
U.S. Pat. No. 5,534,301 to Shutt, issued on Jul. 9, 1996 is hereby incorporated into the present application by reference. This prior art patent provides an up-to-date description of the field of the present invention so as to enable an understanding of the improvements provided by the present invention method. Further, this patent teaches a method for producing a fire-retardant composition including any one or more of ammonium sulfate, monoammonium phosphate, diammonium phosphate, boric acid, aluminum sulfate, sodium tetraborate, ferrous sulfate and zinc sulfate, which as will be described below is relevant to the present invention.
The prior art does not teach the use of alkali borates as liquid flame retardants in cellulose insulation. This has not been accomplished, most likely, because the solubility of boric acid and sodium tetraborate in water, is known to be quite low thereby producing a product with excessive water content.